The purpose of filters in anesthesia circuits is to protect against cross-contamination between the patient and the anesthesia machine. The same machine is used for many different patients. However, the patient connected elements, such as a mask or endotracheal tube, as well as the Y-connector and corrugated tubing, are usually disposed of after a single patient use to prevent contamination. Because it is of utmost importance to protect the patient from as much of the system as possible, it is desirable to place the filter as close as possible to the patient. In the past, it has not been feasible to place the filter between the Y-connector and the patient because the filter was so bulky as to interfere with the anesthesiologist's view and working area at the patient's oral-nasal area.
It is important to understand that to be effective, an anesthesia gas filter must have a filtering efficiency in excess of 95% and have a very low pressure drop across the filter at relatively high gas flow rates. This is so the filter does not provide a strain to the breathing patient, who often has a weakened respiratory system. For use in an anesthesia circuit, a filter must have a pressure drop of less than 0.3 inch of water at flow rates up to 12.5 L gas/minute. In the past, there have been no anesthesia filters that could meet these requirements and still be compact enough not to interfere with the work of the anesthesiologist adjacent the patient's face. For these reasons, the large anesthesia filters of the past have been located remote from the patient. Usually there were separate filters on the inhalation and exhalation conduits.
U.S. Pat. No. 3,556,097 illustrates a typical recycling anesthesia circuit which is connected to a patient through an oral-nasal mask. This mask is connected through a Y-connector to a corrugated inhalation tube 7 and a corrugated exhalation tube 6. In FIG. 1, filter 9 is typically spaced remote from the patient to filter the incoming respiratory gas.
Although FIG. 14 of U.S. Pat. No. 3,556,097 shows an alternate positioning of the filter between the Y-connector and mask, the filter of this patent could not practically be used in this position because of its immense size, causing the patient to rebreathe a large volume of his unfiltered, exhaled gas. It is estimated the body (excluding connectors) of the filter of U.S. Pat. No. ;b 3,556,097 has an internal volume of 100 cc or more. This very large filter would also likely interfere with the anesthesiologist's view and working area in the patient's oral-nasal area.
The applicant is aware of a filter almost identical to that described in U.S. Pat. No. 3,556,097, being marketed under the Foregger name. Upon applicant's information and belief, this Foregger filter is always placed between the Y-connector and the anesthesia machine. Applicant is unaware of any anesthesiologists using the Foregger filter between the patient and the Y-connector. The filter's immense bulk and problem with rebreathing substantial amounts of unfiltered, exhaled gas is probably the reason for this.
U.S. Pat. No. 3,713,440 illustrates another anesthesia system with filters in the typical position in the corrugated tubing between the Y-connector and the anesthesia machine.
A rectangular filter with pleated filtering elements is described in U.S. Pat. No. 3,815,754. This filter has been sold for use with respiratory therapy equipment in which a respiratory therapy machine drives the respiratory gases across the filter. Thus, the pressure drop across the filter is not as critical as an anesthesia machine where the patient's own breathing provides the driving force for the gas across the filter. The filter of this patent is unsuited for placing in an anesthesia circuit between the Y-connector and the patient because its pressure drop and flow rates up to 12.5 L gas/minute is substantially greater than 0.3 inch of water. Also, its immense physical size (approximately the size of the Foregger filter of the previous paragraph) would interfere with the work of the anesthesiologist in the patient's oral-nasal area.